Re: LVM performance vs direct dm-thin

Il 2022-01-30 22:39 Stuart D. Gathman ha scritto:
> I use LVM as flexible partitions (i.e. only classic LVs, no thin pool).
> Classic LVs perform like partitions, literally using the same driver
> (device mapper) with a small number of extents, and are if anything
> more recoverable than partition tables.  We used to put LVM on bare
> drives (like AIX did) - who needs a partition table?  But on Wintel,
> you need a partition table for EFI and so that alien operating systems
> know there is something already on a disk.

Classical (fat) LVs are rock solid, but how do you cope with fast (maybe 
rolling) snapshotting? This is the main selling point of thinlvm.

> Since we use LVs like partitions - mixing with btrfs is not an issue.
> Just use the LVs like partitions.  I haven't tried ZFS on linux - it
> may have LVM like features that could fight with LVM.  ZFS would be my
> first choice on a BSD box.

I broadly use ZFS - and yes, it is a wonderful tools. Than said, it has 
its own gotcha. For example:
- snapshot rollback is a destructive operation (ie: after rollback, you 
permanently lose the current filesystem state);
- clones (writable snapshots) depend on the read-only base image (ie: on 
the original snapshot), which you can not delete until you have its 
clones around.

Moreover, snapshotting/cloning a ZFS dataset (or volume) does not appear 
to be significantly faster then LVM - sometime it requires ~1s, 
depending on the load.

> We do not use LVM raid - but either run mdraid underneath, or let btrfs
> do it's data duplication thing with LVs on different spindles.

I always found btrfs very underperforming when facing random rewrite 
workloads as VMs and DBs. Can I ask your experience?
Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
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